The Mozilla Foundation has issued a patch for a previously undisclosed hole in its popular Firefox browser and is encouraging Firefox users to download the software update as soon as possible.

The nonprofit organisation released Firefox 1.0.2 to fix a buffer overflow vulnerability in a Firefox feature for processing GIF (Graphics Interchange Format) image files. The patch is the second security patch issued in less than a month, but the foundation reassured users that the browser's open source platform is secure, and said it does not know of any active exploits for the hole.

The GIF processing hole was discovered by Internet Security Systems (ISS) and makes Firefox users who are running earlier versions of the browser vulnerable to buffer overflow attack, according to a statement released by the Mozilla Foundation.

ISS discovered the hole in a review of the Firefox source code, which is available on the Internet.

In a statement attributed to Chris Hofmann, the foundation's director of engineering, the discovery of the hole and release of a patch shortly after are evidence that the open source software model is safer and more secure than closed source commercial code, because it is "scoured by thousands" of contributors, developers and professionals, and "not just the company's development team."

In February, the Mozilla Foundation released Firefox 1.0.1 to fix 17 security vulnerabilities in Firefox, including changes to guard against spoofing of web addresses and the security indicator on websites. However, the foundation is not planning to adopt a regular patch release cycle, which Microsoft uses, and will continue to issue updates as they are needed, Hofmann said in a statement.

Firefox has been gaining in popularity since the first full version of the browser was released in November. More than 27 million copies of Firefox have been downloaded since then, pushing Microsoft's Internet Explorer (IE) share of the browser market below 90 percent for the first time in years.

According to statistics released by web tracking company WebSideStory, Firefox installations represent 5.7 percent of the U.S. browser market, while IE controlled 89.9 percent.

However, Hofmann denied that Firefox is becoming a more attractive candidate for hackers as it gains market share.

"There is this idea that market share alone will make you have more vulnerabilities. It is not relational at all. Not being in the operating system and not supporting Microsoft's proprietary Active X are phenomenal advantages to us," he said.